1. Technical Field
This invention generally relates to nondestructive testing and inspection techniques, and deals more particularly with a reference standard and related method used to calibrate ultrasonic measurements of porosity in manufactured parts, particularly those of composite materials.
2. Description of the Related Art
It is often useful to know the porosity of a manufactured part in order to determine its suitability for a particular application or whether it meets a product specification. Porosity is normally defined as the ratio of pores or voids in a part relative to the total volume occupied by the part.
Various technologies have been employed to determine the porosity of the part, including the use of ultrasonic sound in which a beam of ultrasonic energy is directed into the part whose porosity is to be measured. Any voids that may be present in the part attenuate the sound beam in proportion to the porosity of the part. The magnitude of the attenuated beam, which may either pass through the part or be reflected back through the transmission path, is measured to determine the porosity.
In order to accurately measure porosity, porosity reference standards must be used to determine whether the measured porosity is accurate. In the past, porosity reference standards were manufactured in which an attempt was made to produce voids in a reference standard part or block, so that the degree of porosity would be known. This approach was not only time consuming and expensive, but produced results that were not always uniform.
One of the areas in which measurements of porosity is useful is that of parts and structures made from composite materials, such as those used in the aerospace industry. Laminated plies of fiber reinforced polymers have wide spread use in commercial and military aircraft. During the manufacturing stages, multiple plies or layers of fiber reinforced materials are laid up, compacted and then cured to produce homogeneous, rigid structures. If the compaction of the plies is not uniform or inadequate compaction pressure is employed during the production process, voids within the layers may be left that may reduce the structural strength of the part or cause delamination over the service life of the part. Porosity measurement is also important when patching or repairing composite parts since voids can be left along the bondline between the new material of the patch and the old material of the structure.
Accordingly, there is a need for a reference standard used in the ultrasonic measurement of porosity of manufactured parts, particularly those made of composite materials, which is relatively inexpensive to produce and yields accurate, uniform and reproducible results. The present invention is directed toward satisfying this need.